Cohort studies

Question 1

What are the properties of cross sectional studies and how is it useful

Answer 1

exposure status and disease status of individual measured at one point in time
disease prevalence in those with and without exposures or at different exposure levels are compared
useful for health planning

Question 2

How are prevalence ratio, prevalence odds and prevalence odds ratio calculated

Answer 2

prevalence ratio=prevalence of disease in exposed/prevalence of disease in unexposed
Prevalence odds=odds that a diseased person was exposed or unexposed
prevalence odds ratio=ratio of prevalence odds in exposed to prevalence odds in unexposed

Question 3

Pros and cons of analytic approach

Answer 3

Pros: quick and easy to conduct, data collected once, measure prevalence for all factors, multiple outcomes and exposures, generating hypotheses
Cons-difficult to determine time order, unsuitable for rare diseases, reflect determinants of survival and aetiology, unable to measure incidence, difficult to interpret, susceptible to bias

Question 4

Features of cohort studies

Answer 4

prospective, follow up, concurrent, longitudinal
A group of people who share a common experience or condition and who are followed up to determine the incidence of specific disease

Question 5

What are the uses of cohort studies

Answer 5

etiology (risk factors for developing disease?

prognosis (what predicts mortality/disability, what element of care predict other health related outcomes

Question 6

How to calculate relative risk

Answer 6

Number of new cases of disease/total number at risk at start of period
risk in exposed/risk in unexposed=(a/(a+b))/(c/(c+d))

Question 7

How to calculate relative rate (Incidence density)

Answer 7

Number of new cases over a defined period/total ‘person time at risk’ during period

Question 8

How to calculate rate ratio

Answer 8

relative rate in exposed/relative rate in unexposed

Question 9

What is the difference between incidence rate, incidence proportion and incidence density

Answer 9

Incidence rate-measure how rapidly cases are occurring,
Incidence propotion-used when only interested in what happens at end of given period. IP is cumulative
Incidence density-how fast the disease develop

Question 10

Advantage of cohort study

Answer 10

Temporal sequence between putative cause and outcome
Investigation of multiple outcomes associated with a single exposure
Valuable in the study of rare exposures
reduce the risk of survivor bias
allow circulation of incidence rates, relative risks, and confidence intervals

Question 11

Disadvantage of cohort study

Answer 11

Inefficient for rare diseases, require large population, loss to follow up, can be expensive and time consuming

Question 12

Potential biaas sources in cohort studies

Answer 12

Selection bias (sampling bias, ascertainment bias, participation bias)
Information bias (misclassification bias, ecological fallacy)
Confounding
Chance